How Starches Thicken Sauces and Fillings

Are you making a traditional lasagna filled with layers of creamy béchamel? Maybe you are cooking a quick beef and Chinese broccoli stir-fry for dinner with a sauce that's loaded with ginger and garlic? Or perhaps you're considering making a sweet, thick vanilla pastry cream to fill a fruit tart this weekend?

In most cases, the secret to a thick sauce (or filling) that coats food evenly is starch, whether plain flour, cornstarch, tapioca starch, or even arrowroot starch. Here's why these starches do the job so well.

What Is Starch?

Starch is the storage form of glucose sugar in plants, basically a long strand of glucose molecules bound together. Plants make starch from glucose to store energy for later: If a plant ends up in a situation where there aren't enough energy sources around, it can draw energy from the strands of starch it has stored, breaking those strands back down to glucose.

Strands of starch are usually loosely assembled together to form starch granules. So, starch granules are made up of loosely assembled strands of starch, and starch strands are long strings of glucose molecules bound together.

How Does Starch Thicken a Sauce?

In the presence of water and heat, starch granules swell, absorbing water. The movement of water and starch is greatly reduced as the water is trapped in the starch granules and the swelled starch granules press up against each other in the sauce. The reduced movement results in thickening and is the beginning of the gelling process.

As more heat is applied, the starch granules eventually begin to leak, freeing the starch strands from each other. At that point, the liberated starch strands are free to interact with and trap water in a process known as gelling or gelatinization. The starch strands swell and lose their crystalline properties, essentially absorbing surrounding water molecules and preventing movement.

Depending on the starch, the gelling process can occur at temperatures as low as 55ºC (131ºF) or as high as 85ºC (185ºF).

Problems That Can Occur When Thickening with Starches

Not cooking the sauce enough: Undercooking a starch-thickened sauce or filling is a common problem with home cooks. The cooking process is not only essential for achieving the right thickness and set, but it's also important for improving the mouthfeel of thickened sauces, essentially cooking away that raw, "starchy" taste by allowing as much of the starch strands as possible to absorb water and gel. Undercooked starch mixtures are also less stable and tend to weep water when stored.

Cooking the sauce too much: Undercooking a starch-thickened sauce isn't a good idea, but neither is overcooking! Excessive heat exposure can cause the starches to break down too much, essentially lessening the gelling power of the starch, resulting in a thinner sauce.

Sugars and fats (tenderizers): Sugars and fats can disrupt and slow down the rate at which water is absorbed and starches swell, which means it can take much longer for the starch-thickened sauce or filling to gel. Worst-case scenario, these tenderizers can completely prevent gelatinization if a large excess of sugar or fat is present.

Acids: Acids, like heat, can break down large starch molecules into smaller pieces, therefore reducing their ability to gelatinize and thicken a mixture. Acid can also speed up the starch granule swelling process so it happens at a lower temperature, thereby increasing the likelihood that you will overcook the sauce, bringing it past that perfect set point, and resulting in a thinner sauce.

Do you like to thicken sauces with starches? Do you prefer using cornstarch or flour? I'd love to hear what your go-to starch is and how you use it.